To place a call on hold, press the HOLD button

To return the held call, press the held line's button.
If you do not return the held call, it rings back to your phone. 
If you are busy on another call, you hear two tone bursts. 
If the held party hangs up, the call is released

To place a call on Exclusive Hold:
While on a call, press HOLD twice. This places the call on 
Exclusive Hold; no one else will be able to access that line.

To place a call on Automatic Hold:
While on a call, press another extension button to receive/originate a new call. 
You can switch between calls without having to press HOLD

Make sure this feature is programmed on your system; if not, existing calls will drop.